In this paper, we take a Construction Grammar approach to Du Bois' concept of resonance activation. We suggest that the structural mapping relations between juxtaposed utterances in discourse, described in terms of diagraphs in dialogic syntax, can acquire the status of ad hoc constructions or locally entrenched form-meaning pairings within the boundaries of an ongoing conversation. We argue that the local emergence of these ad hoc constructions involves the same cognitive mechanism described for the abstraction of conventional grammatical constructions from usage patterns. Accordingly, we propose to broaden the scope of Construction Grammar to include not only symbolic units that are conventionalized in a larger speech community, but also a dimension of online syntax, i.e. the emergence of grammatical patterns at the micro-level of a single conversation. Drawing on dialogic data from political talk shows and parliamentary debates, we illustrate the spectrum of these ad hoc constructional routines and show their local productivity, which we take as an indication of their (micro-)entrenchment within a given conversation.